Metal oxide sols (ZnO, SnO2, TiO2, ZrO2, Fe2O3, NiO, Ga2O3, MnO2, In2O3, CeO2) were prepared by hydrolysis of ehloro-alkoxo or acetylacetonato complexes, preventing precipitation. The sols were injected in a solution of tetradecene and an aliphatic amine at 160 °C and heated at the resulting temperature for 3 h, resulting in the formation of metal oxide nanoparticles. The crystallization degree of the resulting nanoparticles depended on the particular system, and nanocrystals were obtained for ZnO, SnO2, Fe2O3, Ga2O 3, In2O3, CeO2 systems. The chemical features of the process were investigated on SnO2 and CeO 2 as case systems, by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and nuclear magnetic resonance. The initial hypothesis that the amine present in the injection solution may act at the same time as a catalyst of the inorganic condensation reactions and as a growth stopper by bonding to the surface metal atoms of the nanoparticles, was thoroughly investigated and it resulted that the alkylamine structure and the processing of the starting sol can influence the nanocrystal formation pathway. In particular, bulkier amines favor the formation of smaller nanocrystals, whereas favoring the condensation reactions in the starting sols results in larger nanocrystals. © 2009 American Chemical Society.